Crust of the Earth: A Symposium
The word “crust” is sometimes used to indicate the layers above the Mohorovičić discontinuity, sometimes the lithosphere (as in the present paper), or even the portion of the earth’s mantle in which earthquakes occur. The crust consists of sediments, a “granitic” (“sialic”) layer which is absent in deep ocean basins, a basaltic (“gabbro”) layer which is separated from the deeper “ultrasima” by the Mohorovičić discontinuity. The velocities V of longitudinal and v of transverse waves in km/sec are about V = 6±, v = 3½± in the “sial”, V = 6½ to 7 and v = 3¾± in the “gabbro” layer, and V = 8.2, v = 4¾ in the ultrasima. The depths of the discontinuities are calculated from records of refracted or reflected waves produced by explosions or earthquakes. Results found in different ways for a given region are in good agreement. The depth of the Mohorovičić discontinuity varies between 10± km below sea level in deep ocean basins and 50± km or even more under mountains. Local differences in velocities and depths are pointed out. Some may be spurious and due to errors in calculations of depths and velocities resulting from dipping layers, misinterpretations, and incorrect assumptions about the structure, especially failure to consider effects of unknown or poorly known low-velocity layers. These may produce shadow zones for certain waves or at least very small amplitudes. The difficulty in finding refracted waves with their deepest point in the “gabbro” layer may be caused by such a process. Reflected waves are less affected by low-velocity layers than refracted waves, but pertinent reflection seismograms are scarce. The most likely reason for the low-velocity layers at depths of between 10± and 150± km is a preponderance of a decrease in wave velocity as a consequence of the temperature increase with depth over the increase by pressure. It is not unlikely that such a decrease occurs in each of the crustal layers mentioned above.